Research

Last updated on: 5 May 2015

Unsteady aerodynamics

Unsteady aerodynamics of airfoils, wings, or blades experiencing large and/or rapid excursions in flow angle (angle of attack) and/or flow velocity is of interest in many applications in nature and engineering. Such flows are encountered in flapping flight of birds and insects, micro air vehicles (MAVs), rotorcraft, wind turbines, and other engineering devices. Our research aims to develop fast low-order prediction methods suitable for design, simulation, and control. Because wings undergoing pitch, plunge, and surge motions have complex flow phenomena such as apparent-mass effects, vortex shedding, and large-scale separation, the development of low-order methods is challenging. We draw on results from high-order computational fluid dynamics (CFD) and experimental studies to phenomenologically augment theory to develop efficient low-order techniques. In particular, we have developed an inviscid criterion for leading-edge vortex (LEV) formation. This criterion, based on the criticality of what we call the Leading-Edge Suction Parameter (LESP), has been incorporated in an unsteady airfoil theory method with discrete-vortex shedding to predict the initiation, growth, and termination of intermittent LEV shedding from rounded leading-edge airfoils. Traditional discrete vortex methods can handle only continuous vortex shedding, which is correct only for sharp edges, or have some ad-hoc criteria for switching the shedding on or off. Our LESP criterion, on the other hand, is motion independent and can be tracked during the unsteady flow calculation to dynamically determine the “start” and “stop” of LEV formation. Current efforts aim to extend this work to 3D finite wings and helicopter rotor blades.

Sponsors

  • U.S. Air Force Office of Scientific Research, grant FA 9550-13-1-0179, April 2013-March 2016
  • U.S. Army Research Office, grant W911NF-13-1-0061, March 2013-February 2016
  • U.S. Air Force Office of Scientific Research, grant FA 9550-10-1-0120, March 2010-March 2013

Students

  • Greg McGowan (PhD, 2008)
  • Kiran Ramesh (PhD, 2014)
  • Sachin Aggarwal (MS, 2013)
  • Shreyas Narsipur (PhD candidate)
  • Yoshikazu Hirato (PhD candidate)

Collaborations

  • Prof. Jack Edwards, North Carolina State University
  • Dr. Michael OL, US Air Force Research Laboratory, Wright-Patterson Air Force Base
  • Dr. Kenneth Granlund, US Air Force Research Laboratory, Wright-Patterson Air Force Base
  • Dr. Joseba Murua, University of Surrey, UK

Post-stall aircraft aerodynamics and flight dynamics

The aerodynamics and flight dynamics of aircraft configurations at small angles of attack are generally well understood and can be modeled using fast low-order methods. At high angles of attack at and beyond stall, however, flow separation over portions of the lifting surfaces makes it difficult to predict the aerodynamics of the aircraft. With the increasing demand for accurate and reliable flight simulation capability at these conditions (for training pilots in recognizing and recovering from stall, spin, and upset conditions), there is a need for fast aerodynamic prediction methods that can be used in real-time simulation. Our post-stall prediction method is based on a decambering approach, that we first developed in 2003-06. Given the instantaneous aerodynamic inflow angles and the angular-velocity components, the post-stall method predicts the aerodynamic forces and moments on the aircraft. Inputs to the aerodynamic prediction method include planform-geometry details and lift, drag, and moment curves for all the airfoil sections including post-stall information. Our current efforts are in using the decambering approach in a fast aerodynamic prediction method that can be used at every time step of the flight dynamics simulation at pre- and post-stall conditions. Because of a dearth of post-stall experimental data for validation, high-order computational (CFD) studies tailored for validation have been initiated. Our current effort uses the NASA TetrUSS CFD package for this purpose.

Sponsors

  • NASA Langley Research Center, 2000-03
  • NASA Langley Research Center, National Institute of Aerospace, 2011-14

Students

  • Rinku Mukherjee (PhD, 2004)
  • Matthew Sutton (MS, 2010)
  • Justin Petrilli (MS, 2013)
  • Ryan Paul (PhD, 2015)
  • Kristen Patrick (MS, 2014)
  • Pranav Hosangadi (MS candidate)

Collaborations

  • Prof. Fen Wu, North Carolina State University
  • Dr. Neal Frink, NASA Langley Research Center
  • Mr. Gautam Shah, NASA Langley Research Center
  • Dr. Joseba Murua, University of Surrey, UK

Adaptive aerodynamics and aerodynamic sensing

It is believed that birds take advantage of their variable-geometry wings to achieve near-optimum spanwise lift distributions for various flight conditions. In contrast, most current-day aircraft with rigid wings have less flexibility to adapt their wing geometry, except for the use of high-lift devices. Over the past couple of decades, there has been renewed interest in the use of adaptive or morphing lifting surfaces in an effort to optimize an aircraft for several flight conditions. Airfoil adaptation with trailing-edge “cruise” flaps can be used to tailor an airfoil for multiple flight conditions. Wing-shape adaptation via spanwise camber variation can be used not only to reduce profile and induced drag, but also to reduce wing weight with structural-load alleviation at maneuvering conditions by redistributing the spanwise load distribution. Overall, these benefits can lead to reduced fuel burn and lower emissions. In a series of projects, we studied the benefits of multiple trailing-edge flaps on wings and aircraft configurations, including tailless and three-surface aircraft. We developed methods for determining optimum flap-angle scheduling for simultaneously reducing profile and induced drag while maintaining constraints on lift and pitching moment. We also developed approaches for aerodynamic sensing (in-flight determination of the lift coefficient of the wing section) using a few pressure measurements in the leading-edge region of the section. We showed in wind-tunnel experiments that the approach can also be used for automatically setting trailing-edge flaps on a wing at the optimum angles for any flight condition.

Sponsors

  • NASA Langley Research Center, 2004
  • KalScott Engineering through STTR from NASA Dryden Research Center, 2008
  • General Electric (Wind and Water Power), 2013

Students

  • Christopher McAvoy (MS, 2002)
  • Victor Vosburg (MS, 2004)
  • Rachel King (PhD, 2005)
  • Jeffrey Jepson (PhD, 2006)
  • Edward Shipley (MS, 2005)
  • Aaron Cusher (MS, 2005, PhD, 2009)
  • Hidehiro Segawa (MS, 2007)
  • Craig Cox (PhD, 2008, co-advised with Dr. Chuck Hall)
  • Aditya Saini (MS, 2014, PhD candidate)

Collaborations

  • Prof. Xiaoning Jiang, North Carolina State University

Airfoil, wing, configuration, and rotor/propeller aerodynamics

Our research in aerodynamic design is in the development of design methods and their application to the design of flight vehicles, ground vehicles, and wind power devices. We have developed design approaches and methods for natural-laminar-flow (NLF), low Reynolds number, and high-lift airfoils. Special emphasis has been given to matching an airfoil to the application (like a particular aircraft) either through closed-form expressions or by connecting the airfoil design-parameter space with simulation of the system’s performance. For design of wings and configurations, we have developed approaches for tailoring lift distributions on wings and configurations to simultaneously minimize induced and profile drag subject to constraints on wing bending moment, aircraft trim, and stall behavior. These approaches have been used by Dr. Gopalarathnam and several students for design of airfoils and wings for general aviation aircraft, radio-control aircraft, race cars, and America’s Cup yachts. Design approaches have also been developed for horizontal-axis and vertical-axis wind turbines, some of which have been studied in wind-tunnel experiments.

Sponsors

  • Oracle-BMW Racing (2003 America’s Cup team), 2001-02
  • Cirrus Design, 2006
  • Flexsys, Inc, 2006-08
  • KalScott Engineering, 2005
  • Mr. John Ketcham, private sponsor via research donations, 2010-12

Students

  • Christopher McAvoy (MS, 2002)
  • Jeffrey Jepson (PhD, 2006)
  • Seth Short (MS, 2008)
  • Wolfgang Sanyer (MS, 2011)
  • Sriram Pakkam (MS, 2011)
  • Aditya Balu (MS, 2011)
  • Dhruv Rathi (MS, 2012)
  • Rajmohan Waghela (MS candidate)

Bio-inspired aerodynamics and flight

Ideas from animal flight provide inspiration for engineering. Two research efforts in this area are described here. (1) We have studied the aerodynamics of formation and ground-effect flight, which are used by migrating birds to reduce induced drag. We developed exact solutions for lift distributions and downwash behind wings flying in formation and in ground effect. The optimum lateral separation between wings flying in formation leads to the vee shapes observed in bird formation flight. We showed that for large formations, in or out of ground effect, multiple local optima are seen for the lateral separation between wings. The presence of these local minima with nearly the same drag might explain the considerable variations and imprecision in lateral separation that are often observed in bird formations. (2) In another research effort, we conducted a series of wind tunnel experiments to study the effect of feather-like lift-enhancing effectors on the upper surface of an airfoil. These effectors or flaps are also sometimes referred to as “hairy flaps” in the literature. When operating at and beyond the clean airfoil’s stall angle, the free effector automatically deploys to progressively higher angles with increasing angles of attack. This slows down the rapid upstream movement of the separation point and avoids the severe reduction in the lift coefficient and an increase in the drag coefficient that are seen on the clean airfoil at the onset of stall. Thus, the effector postpones the stall by 4–8 degrees and makes the stall behavior more gentle. The benefits of using the effector could include care-free operations at high angles of attack during perching and maneuvering flight, especially in gusty conditions.

Students

  • James Frazier (MS, 2002)
  • Rachel King (PhD, 2005)
  • Joe Johnston (MS, 2011)